Time-dependent phase separationhransition was observed in aqueous solutions of poly(N-isopropylmethacrylamide) in the temperature range 38-42 "C. The time before the second phase appears is a function of temperature and may reach up to several hours.
A series of copolymers of N-(2-hydroxypropyl)methacrylamide were prepared, which contained side chains of the general formula -Gly-X-Y-NAP, where Gly. . glycine; X. . glycine, alanine, 8-alanine, valine, leucine, isoleucine, phenylalanine; Y . . phenylalanine or tyrosine; NAP. . p-nitroanilide, the latter modelling biologically active compounds. The rates of chymotrypsin-catalyzed hydrolysis of p-nitroanilide groups at pH = 8,O and 25 "C were determined over a range of substrate concentrations to derive values for k,, and KM. The results allowed us to determine the influence of the structure of side chains on the rate of cleavage of Y-NAP. The increase in the susceptibility to chymotrypsin attack with an increasing spacing of the Y-NAp residue from the backbone of the polymer chains is demonstrated by comparing the kinetic data of copolymers containing -Gly-Gly-Phe-Phe-NAP, -Gly-Gly-Phe-NAp and -Gly-Phe-NAp side chains. Results obtained with a-chymotrypsin were compared with the cleavage of the above polymer substrates with chymotrypsin covalently bound to a copolymer of N-(2-hydroxypropyl)methacrylamide. a) Presented at the 25'h IUPAC Symposium on Macromolecules, Mainz, FRG, September 1919. 0250-9733/81/0182/0799/$03 .OO 6 NO 2 3 , 4 , 6 -1 2 Polymeric substrate 30 -43 for R' = H, substrate is denoted by a, e.g. 30a for R' = OH, substrate is denoted by b, e.g. 30b Comonomer I R? R3 3 4 6 7 8
The phase separation and its thermohysteresis in dilute aqueous solutions of polymeric components of potential drug release systems (homopolymers and copolymers of N-isopropylacrylamide, N-isopropylmethacrylamide, N-propylmethacrylamide, N-sec-butylmethacrylamide, and N-(2-hydroxypropyl)methacrylamide) was studied, both on heating and cooling. Plots of light transmittance vs temperature were constructed and the parameters characterizing them were correlated with polymer structures. Qualitative information was obtained on the rate of formation of the concentrated phase on heating and its disappearance on cooling. Attention has been drawn to the improper identification of the cloud-point temperature, measured at an arbitrary concentration, with the lower critical solution temperature (LCST) as is frequently found in papers dealing with biomedical applications of thermosensitive polymers.
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